#define  _CRT_SECURE_NO_WARNINGS


struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(nullptr) {
    }
};

class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        ListNode* fast = pHead;
        ListNode* slow = pHead;
        while (fast && fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;
            if (fast == slow)
            {
                slow = pHead;
                while (fast != slow)
                {
                    fast = fast->next;
                    slow = slow->next;
                }
                return fast;
            }
        }
        return nullptr;
    }
};